The present invention relates to an improved polarizer comprising a polarizer element and an array of tapered waveguides in substantial contact with the polarizer element.
Optical waveguides, also known in the art as light transmissive devices or lightguides, find application in display devices, such as for example projection display devices, off screen display devices, and direct view displays. See for example U.S. Pat. Nos. 3,218,924 and 3,279,314 to Miller and U.S. Pat. No. 4,767,186 to Bradley, Jr. et al. Such displays are used in a wide range of applications including computer terminals, airplane cockpit displays, automotive instrument panels, televisions, and other devices that provide text, graphics, or video information.
Such displays usually consist of a laminate of a liquid crystal element between two pieces of glass or plastic film. The inner surface of the glass is rubbed or otherwise treated to induce a preferred orientation of the liquid crystalline molecules near its surface. The operation of a liquid crystal device under a back lighting arrangement requires that polarizers be employed on the inner (back) and outer sides. The direction of orientation of the liquid crystals together with the specification of a "normally on" or "normally off" device dictates the required orientation of the inside and outside polarizers. It is generally desirable to spread light anisotropically from a display device and thus, send increased amounts of light into the horizontal plane (zx) and less into the vertical plane (zy).
One embodiment of a direct view display device based on a liquid crystal material is the twisted nematic (TN) liquid crystal display device. In this embodiment, a nematic liquid crystal medium is sandwiched between substrates which are treated so as to cause spontaneous alignment of liquid crystal molecules parallel to the plane of the substrate. If the two substrates are oriented so that alignment at each substrate differs by 90 degrees, then the liquid crystal molecules will undergo a 90 degree orientation change throughout the thickness of the medium. For reasonable spacing of the substrates (typically about 5 micrometers), this configuration has the property of rotating the polarization of light incident normal to the plane of the substrates by 90 degrees. If an electric potential is applied between the substrates (typically a few volts), then the order of the liquid crystal molecules is altered. In the presence of the potential, the molecules will tend to align perpendicular to the substrate and the 90 degree rotation will be destroyed. Thus the polarization of light incident normal to the surface of the substrate will be unaltered in the presence of the electric potential. Based on these principles, direct view twisted nematic (TN) liquid crystal devices may be constructed which are either normally black (NB) in the absence of the potential or which are normally white (NW) in the absence of the potential.
As an example, a typical normally white display consists of a TN liquid crystal cell as described above fitted with polarizers elements on either side of the cell so that unpolarized light incident normal to the plane of the device is linearly polarized as it enters the device. The polarization is rotated by 90 degrees as the light transverses the cell. The light is then transmitted by the second polarizer element which is oriented at 90 degrees to the first polarizer element. Thus in the absence of an electric potential, light incident normal to the device is transmitted through the structure. When an electric potential is applied between the substrates, the medium no longer rotates the polarization by 90 degrees. Thus light which is incident normal to the structure is rejected by the second polarizer element and not transmitted. In this way, image information contained in the pattern of applied electric potential is presented as a decrease in light as seen by the viewer. This is the operating principle of a simple twisted nematic normally white (NW) display.
It is generally a shortcoming of such display devices that insufficient light is projected into higher angles and thus, viewability is limited to a narrow angular range around the normal to the plane of the liquid crystal device, i.e., the z-axis. Another shortcoming is that when viewed at high angle to the normal of the plane, the quality of the image is degraded, i.e., it exhibits undesirable color shift, limited gray scale, low contrast, and low sharpness.
As such, the need exists in the art for a display device wherein viewability around the normal to the plane of the liquid crystal device is improved and the quality of an image when viewed at high angle is also improved.